I learnt hybridization between somatic cells and ES cells yields mononuclear tetraploid ES cell lines. I wonder why the nuclei fuse... Given that endoplasmic reticulum is a network around the nucleus and continuous with the outer nuclear membrane, does this mean that the two ERs fuse as well? And how do the nuclei manage to find each other?
I'm probably overthinking this, but I also recall that microtubules and microtubule motors prevent the ER from collapsing to the center of the cell, i.e. they drag it away from the cell center. So if each nucleus is "repulsing" its ER, shouldn't it also repulse the ER of the other cell? Why do the nuclei near each other rather than pushing themselves away?

There are some excellent questions here! The first thing to say is that in general fusing cells is a relatively inefficient process and that the detailed mechanism(s) underlying it are unknown. From some of the earlier experiments polyethylene glycol (PEG)-induced cell fusion involves dehydration of membrane surfaces forcing close contact between membranes, and it appears that the plasma membrane AND then cytoplasmic compartments become one (http://jcb.rupress.org/content/96/1/151.long; http://www.tandfonline.com/doi/pdf/10.1080/096876899294508), and I would imagine that this includes the ER.

The ES-somatic cell hybrids that result from fusion (using PEG, for example) appear to adopt the transcriptional profile of ESCs with apparent silencing of many (but not all) somatic genes (http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4030652/). I think that in some cell fusion hybrids there may be some evidence for the organelles of one cell adopting the ‘shape’ and polarity of the other, which may involve alterations in the cytoskeleton - I don’t know if this happens in ES-somatic cells. PEG itself induces cytoskeletal reorganisation in many cells (e.g., see - http://ncbi.nlm.nih.gov/pmc/articles/PMC3736723/). I also don’t know how the various organelles ‘find’ each other - perhaps there is a high level of ‘intermixing’ which leads to unviable hybrids, and the ‘specificity’ is based on physical/chemical interactions that underpins viability/growth. This article - http://www.sciencedirect.com/science/ar … 5492900028 - shows the ultrastructural changes during PEG fusion of two cells (including a nice figure (3) showing intermediate nuclear fusion and possible mitochondria-nucleus fusion).

Last edited by Steve Lolait (16th Jan 2016 09:36:15)